This method is negated by any wallet stealer since both the thief and victim would have the private key. Given that info, how do you prove that you were ever the lawful owner of the BTC at that time? Only thing I can think of is maybe a receipt of some kind (exchange logs?) and that doesn't take into account the fact that the BTC could have been traded willingly off network by sending the private key to the buyer (or thief [I can know as a 3rd party{you can't either}]).
As I mentioned in my blog which started this discussion, proving original ownership of the privacy key will be difficult and certainly will be subject to a finding of fact in any litigation.
It's not just difficult it's likely impossible. Please describe a scenario in which such proof is even civil court sufficient which is "by the preponderance of evidence". You can probably forget the criminal aspect of "beyond any reasonable doubt" but if you are up to it you can describe that as well.
Preponderance is basically a 51% deal. If it's more likely than not that you own the private key, you prevail on that standard. So if you can actually produce the key, that would prove that you actually own it to any reasonable person, especially if you also, as you would, swear under oath that you are the rightful owner of that key.
Civil court cases quite often turn into "swearing contests" just like this, with one person saying one thing and the other side saying another, and the fact-finder deciding which of them is telling the truth.
But actually possessing the private key is a lot stronger than just saying you are the rightful owner of it. There's no "beyond any reasonable doubt" in a civil case, which a recovery action would usually be. The burden is preponderance of evidence, and actually producing the private key while the other side is incapable of doing so (since it doesn't own it) would shift the burden to the other side to prove you were
not the rightful owner.